implement option for run length compression in resource profiles

merlin-driver/src/main/java/gov/nasa/jpl/aerie/merlin/driver/engine/SimulationEngine.java

@@ -466,19 +466,20 @@ public static SimulationResults computeResults(
 
       final var name = id.id();
       final var resource = state.resource();
+      final boolean allowRLE = resource.allowRunLengthCompression();
 
       switch (resource.getType()) {
         case "real" -> realProfiles.put(
             name,
             Pair.of(
                 resource.getOutputType().getSchema(),
-                serializeProfile(elapsedTime, state, SimulationEngine::extractRealDynamics)));
+                serializeProfile(elapsedTime, state, SimulationEngine::extractRealDynamics, allowRLE)));
 
         case "discrete" -> discreteProfiles.put(
             name,
             Pair.of(
                 resource.getOutputType().getSchema(),
-                serializeProfile(elapsedTime, state, SimulationEngine::extractDiscreteDynamics)));
+                serializeProfile(elapsedTime, state, SimulationEngine::extractDiscreteDynamics, allowRLE)));
 
         default ->
             throw new IllegalArgumentException(
@@ -630,11 +631,24 @@ private interface Translator<Target> {
     <Dynamics> Target apply(Resource<Dynamics> resource, Dynamics dynamics);
   }
 
+  private static <Target>
+  void appendProfileSegment(ArrayList<ProfileSegment<Target>> profile, Duration duration, Target value,
+                            boolean allowRunLengthCompression) {
+    final int s = profile.size();
+    final ProfileSegment lastSeg = s > 0 ? profile.get(s - 1) : null;
+    if (allowRunLengthCompression && lastSeg != null && value.equals(lastSeg.dynamics())) {
+        profile.set(s - 1, new ProfileSegment<>(lastSeg.extent().plus(duration), value));
+    } else {
+      profile.add(new ProfileSegment<>(duration, value));
+    }
+  }
+
   private static <Target, Dynamics>
   List<ProfileSegment<Target>> serializeProfile(
       final Duration elapsedTime,
       final ProfilingState<Dynamics> state,
-      final Translator<Target> translator
+      final Translator<Target> translator,
+      final boolean allowRunLengthCompression
   ) {
     final var profile = new ArrayList<ProfileSegment<Target>>(state.profile().segments().size());
 
@@ -643,18 +657,21 @@ List<ProfileSegment<Target>> serializeProfile(
       var segment = iter.next();
       while (iter.hasNext()) {
         final var nextSegment = iter.next();
-
-        profile.add(new ProfileSegment<>(
-            nextSegment.startOffset().minus(segment.startOffset()),
-            translator.apply(state.resource(), segment.dynamics())));
+        appendProfileSegment(profile,
+                             nextSegment.startOffset().minus(segment.startOffset()),
+                             translator.apply(state.resource(), segment.dynamics()),
+                             allowRunLengthCompression);
         segment = nextSegment;
       }
 
-      profile.add(new ProfileSegment<>(
-          elapsedTime.minus(segment.startOffset()),
-          translator.apply(state.resource(), segment.dynamics())));
+      appendProfileSegment(profile,
+                           elapsedTime.minus(segment.startOffset()),
+                           translator.apply(state.resource(), segment.dynamics()),
+                           allowRunLengthCompression);
     }
 
+    profile.trimToSize();
+
     return profile;
   }
 

merlin-framework/src/main/java/gov/nasa/jpl/aerie/merlin/framework/Registrar.java

@@ -13,18 +13,34 @@
 import java.util.function.UnaryOperator;
 
 public final class Registrar {
+
+  /**
+   * Whether to allow run length compression when saving resource profiles at the end of simulation by default.
+   *
+   * This compression is lossless in terms of the overall shape of the profile, but it will combine adjacent profile
+   * segments with the same value, thus obscuring the fact that multiple resource samples (again, all returning the same
+   * value) were taken within the segment.
+   */
+  private static final boolean ALLOW_RUN_LENGTH_COMPRESSION_BY_DEFAULT = false;
+
   private final Initializer builder;
+  private boolean allowRunLengthCompression = ALLOW_RUN_LENGTH_COMPRESSION_BY_DEFAULT;
 
   public Registrar(final Initializer builder) {
     this.builder = Objects.requireNonNull(builder);
   }
 
+  public void allowRunLengthCompression(final boolean allow) {
+    this.allowRunLengthCompression = allow;
+  }
+
   public boolean isInitializationComplete() {
     return (ModelActions.context.get().getContextType() != Context.ContextType.Initializing);
   }
 
   public <Value> void discrete(final String name, final Resource<Value> resource, final ValueMapper<Value> mapper) {
-    this.builder.resource(name, makeResource("discrete", resource, mapper.getValueSchema(), mapper::serializeValue));
+    this.builder.resource(name, makeResource("discrete", resource, mapper.getValueSchema(), mapper::serializeValue,
+                                             allowRunLengthCompression));
   }
 
   public void real(final String name, final Resource<RealDynamics> resource) {
@@ -46,14 +62,16 @@ private void real(final String name, final Resource<RealDynamics> resource, Unar
                 "rate", ValueSchema.REAL))),
             dynamics -> SerializedValue.of(Map.of(
                 "initial", SerializedValue.of(dynamics.initial),
-                "rate", SerializedValue.of(dynamics.rate)))));
+                "rate", SerializedValue.of(dynamics.rate))),
+            allowRunLengthCompression));
   }
 
   private static <Value> gov.nasa.jpl.aerie.merlin.protocol.model.Resource<Value> makeResource(
       final String type,
       final Resource<Value> resource,
       final ValueSchema valueSchema,
-      final Function<Value, SerializedValue> serializer
+      final Function<Value, SerializedValue> serializer,
+      final boolean allowRunLengthCompression
   ) {
     return new gov.nasa.jpl.aerie.merlin.protocol.model.Resource<>() {
       @Override
@@ -82,6 +100,11 @@ public Value getDynamics(final Querier querier) {
           return resource.getDynamics();
         }
       }
+
+      @Override
+      public boolean allowRunLengthCompression() {
+        return allowRunLengthCompression;
+      }
     };
   }
 

merlin-sdk/src/main/java/gov/nasa/jpl/aerie/merlin/protocol/model/Resource.java

@@ -16,4 +16,18 @@ public interface Resource<Dynamics> {
    * this resource. In other words, it cannot depend on any hidden state. </p>
    */
   Dynamics getDynamics(Querier querier);
+
+  /**
+   * After a simulation completes the entire evolution of the dynamics of this resource will typically be serialized as
+   * a resource profile consisting of some number of sequential segments.
+   *
+   * If run length compression is allowed for this resource then whenever there is a "run" of two or more such segments,
+   * one after another with the same dynamics, they will be compressed into a single segment during that serialization.
+   * This does not change the represented evolution of the dynamics of the resource, but it loses the information that a
+   * sample was taken at the start of each segment after the first in such a run.  If a mission model prefers not to
+   * lose that information then it can return false here.
+   */
+  default boolean allowRunLengthCompression() {
+    return false;
+  }
 }